Sparkplugs in internal combustion engines, and igniters in gas turbines and jet engines, are used for igniting combustible mixtures of gases and vapors.
Typically, sparkplugs used in internal combustion engines include a ceramic insulator, which surrounds a central electrode. The end portion of the sparkplug, including the end of the ceramic insulator, is exposed to the interior of the combustion chamber of the engine. During normal operation of the engine, there are times when operating conditions contaminate the ceramic insulator of the sparkplug. Often, this contamination takes the form of electroconducting coke. This contamination can occur, for example, during engine idle when the engine has just been started and the fuel mixture is enriched, when the weather is cold, and when the engine is not properly tuned. It has been found that even when an engine is properly tuned, carbonization of the sparkplug can still take place, for example, when driving on a cold engine or using an enriched fuel mixture. The contamination can also take the form of lead-oxide, due to the presence of lead in leaded gasolines.
The contamination of the sparkplug's ceramic insulator can interfere with the operation of the sparkplug. For example, contamination with electroconductive coke or lead oxide reduces the sparking ability of the sparkplug, because the electrical current dissipates through the contaminant layer to the metal casing of the sparkplug.
Contamination of the insulator can shorten the life span of the sparkplug and make it difficult to start the engine, especially during cold weather. It can also degrade fuel economy by increasing gasoline consumption. Furthermore, it can result in unstable ignition or combustion, which can damage the catalytic converters used as emission control equipment on modern automobiles. This occurs as a result of the discharge of incompletely combusted products into the exhaust system. As catalytic converters are expensive automotive components, it would be desirable to minimize damage to them.
For the reasons set forth above, the demand for sparkplugs and igniters which are resistant to contamination is increasing. These sparkplugs would have extended lifespans. Various solutions for reducing the contamination of the sparkplug insulator have been proposed. However, all of these solutions have significant disadvantages.
One method of reducing sparkplug contamination is by using a sparkplug which has a lower hotbulb number than that recommended by the engine manufacturer. This method has serious deficiencies. While it may reduce contamination of the insulator, the central electrode could be damaged or destroyed during engine operation. If this were to occur, fragments of the central electrode could damage the cylinder/piston system of the engine and thus, ruin the engine.
Another method for preventing contamination of a sparkplug's central insulator is disclosed in U.S. Pat. No. 4,937,484. This patent discloses a heat-resistant coating for a sparkplug's ceramic insulator. The coating is formed from a solution of silicone oil, paraffin and ozokerite which are combined in a manner that increases the coating's viscosity. This coating is designed to be permanent and durable. However, this coating has a disadvantage, in that it increases the temperature of the end portion or tip of the sparkplug beyond the range for which the sparkplug was designed. This can reduce the lifespan of the sparkplug during ordinary use.
U.S. Pat. No. 5,174,298 discloses an ablative coating for a spark plug insulator. This coating provides protection from coke formation during the initial life of the vehicle. However, the coating is designed to be relatively quickly degraded by engine operation, and eventually burns off of the ceramic insulator, exposing it to contamination. The ablative coating is the result of the application of an aqueous mixture of a suspension material selected from kaolin, ball clay, bentonite, quartz, zirconia, and combinations thereof and a binder selected from colloidal silica, colloidal alumina, methyl cellulose, polyvinyl alcohol, and combinations thereof.
The most prevalent method used to prevent carbonization of the sparkplug is through the use of deep hydrocarbon oxidation catalysts as the central or side electrode. The catalyst is either the material used to form the electrode or a metallic coating thereon. These catalytic materials are noble metals, such as platinum, palladium, silver or alloys thereof.
Generally, platinum is the preferred material for these sparkplugs and platinum electrode sparkplugs are commercially available from BOSCH and CHAMPION. These sparkplugs are prepared by introducing a platinum wire into the mold for manufacturing the central electrode. Then, powdered ceramics are added to the mold and the mixture is dried and sintered. After cooling, the material is mechanically treated to provide a central electrode with a platinum cathode that serves as the deep hydrocarbon oxidation catalyst. The catalytic action of the electrode prevents contamination of the insulator.
One method of coating the central sparkplug electrode stem with catalytically active noble metals, such as silver, palladium or platinum, is disclosed in German Patent 3918272. The primary disadvantage of this method is its high cost. The use of platinum and other noble metals adds significantly to the cost of materials used to form the sparkplug. For example, the electrode may require several grams of platinum in each sparkplug. In addition, special ceramics, which have a coefficient of thermal expansion that is compatible with platinum, are also required.
It would be desirable to coat the ceramic surface of the central electrode with a relatively inexpensive, non-conductive deep oxidation catalyst. However, the use of other catalysts, such as electrically insulating metal oxide catalysts, has not been viable, because the methods for production of these coatings are ineffective or commercially impractical for use on sparkplugs.
One known method for the formation of metal oxide coatings is via the electrochemical sedimentation of metal oxides from solution. This method can not be used to coat sparkplug insulators because the ceramic material is not electrically conductive, a requirement of this method.
Another method of forming metal oxide coatings is a sputtering process. In this process, a metal oxide powder is vaporized, using, for example, laser, plasma, flame or detonation methods. The vapors are carried in a gas carrier until they reach the surface to be coated. This surface is generally relatively cool, so that the vaporized metal oxide powder condenses on the surface.
The sputtering process is impractical and expensive to use for coating sparkplugs, making it commercially unreasonable. It requires the use of specialized manufacturing equipment to maintain the necessary vacuum conditions. These requirements increase the cost and difficulty of manufacturing the sparkplug. Further, the materials used in the sputtering process, solid solutions of oxides, are expensive.
Detonation spraying is another possible method that could be used to produce coatings. However, this method requires a large amount of catalyst material and specialized manufacturing equipment.
Another potential method for the production of catalytic coatings is by deposition of metal oxides from aqueous solutions, followed by heat treatment. However, this method requires the use of a binder, such as phosphoric acid, in the metal oxide solution. The presence of phosphorus containing compounds in the catalytic layer reduces its catalytic activity.
A further potential method is chemical vapor deposition (CVD) However, CVD requires the use of a carefully developed system of safety equipment, as toxic components are used in this process. These materials include .beta.-diketones and metal-carbonyl complexes.
Another known method for the deposition of a metal oxide coating on a substrate is described in U.S.S.R. Inventors Certificate No. 923232. This process uses a single trivalent metal salt formed by adding the metal to a solution of carboxylic acids. The salt is applied to a substrate and subjected to a temperature of 500 to 600.degree. C. for 20 to 30 seconds in a nonoxidizing atmosphere.
This process suffers from several drawbacks. First, it requires the use of a special furnace that contains a nonoxidizing atmosphere. In addition, the short heat treatment time produces a coating that has internal strains, leading to the lack of a well defined crystalline structure and creating a highly absorptive coating that can absorb water, gases or hydrocarbons. This absorption can make the coating conductive. Furthermore, the use of a single metal in the coating is undesirable because a number of metals, for example zirconium, do not form resistant coatings without additional additives. This would create weak coatings. Finally, this process only uses trivalent and tetravalent metals. As a result, desirable materials, such as nickel, copper and gold, can not be used therein.
As noted above, the above-referenced conventional coating processes, which have not been used to coat sparkplugs, suffer from a variety of disadvantages that would make them undesirable for the commercial-scale production of coated sparkplugs. In addition, the coatings produced by these methods are relatively thick. This could lower the adhesion between a coating formed using these methods and the sparkplug, and could cause scaling or flaking off of the coating. Further, the above methods require additional procedures to remove the coating from areas of the surface where no coating is desired. Finally, all of the above methods have high material consumption, increasing their cost.
Accordingly, it would be desirable to provide a low-cost, convenient method to form a metal oxide coating on the central insulator of a sparkplug.
One object of the present invention is to provide a commercially viable method for the production of sparkplugs which are resistant to carbonization or lead-oxide buildup.
Another object of the present invention is to provide a sparkplug that is resistant to carbonization or lead oxide build up, due to the presence of a metal oxide coating on the central ceramic insulator insulator of the plug. A related object of the present invention is to provide a sparkplug that will provide longer life and greater efficiency when used with leaded gasolines, which are still in widespread use in many countries outside the United States.
A further object of the present invention is to provide a sparkplug with an extended lifespan over a variety of operating conditions, without the need to use expensive components such as platinum.
A further object of the present invention is to provide a method for depositing a metal oxide coating onto a substrate, including the ceramic insulator of a sparkplug, that is relatively easy to carry out, without using complicated equipment or materials, and using relatively non-toxic materials as compared to prior art methods.